Methods of determining gloss

Abstract

Glossiness is evidenced by almost every object and is to be attributed to specular reflection. Specular reflection occurs at the surfaces of reflecting objects; and, because of the diversities of minute surface structure, many kinds of glossy appearance result. An attempt to classify these glossy appearances has led to the description of six different kinds of gloss: (1) specular gloss, identified by shininess; (2) sheen, identified by surface shininess at grazing angles; (3, contrast gloss, identified by contrasts between specularly reflecting areas of surfaces and other areas; (4) absence-of-bloom gloss, identified by the absence of reflection haze or smear adjacent to reflected high lights; (5) distinctness-of-reflected-image gloss, identified by the distinctness of images reflected in surfaces; and (6) absence-of-surface-texture gloss, identified by the lack of surface texture and surface blemishes. In describing the appearances of objects, one commonly distinguishes between the effects due to two types of reflection; glossiness, on the one hand, may be correlated with specular, or surface reflectance; the degree of lightness or darkness may, on the other hand, be correlated with diffuse reflectance usually occuring within the pigmented and scattering media beneath these surfaces. Specular reflection is evidenced by light preponderantly reflected in the direction of mirror reflection, whereas diffuse reflection is evidenced by light scattered in all directions by the reflecting object. Unfortunately for the purposes of gloss and reflectance measurement, the effects of specular reflection and diffuse reflection cannot be completely separated. The gloss of a surface cannot, in the general case, be defined in any simple way that permits quantitative measurement. Data which describe the directional distribution of light reflected by surfaces illuminated under specified conditions furnish the fundamental physical basis for describing gloss. However, such distribution data are cumbersome and involved and include the effects of both diffuse and specular reflection . It is because these goniophotometric (reflection distribution) data are unwieldy that, in the past, devices for measuring gloss have been developed by simple empirical means. By trial and error, methods have been found to measure the gloss of particular types of materials exhibiting particular types of gloss . The article suggests that the designer of a prospective gloss meter should deter-mine from goniophotometric data taken on representative samples what differ-ences in apparent reflectance are most characteristic of the differences in glossiness observed visually. That is, gloss-meter design will have considerably less of the trial-and-error element when goniophotometric data are used to indicate the most pertinent reflectance measurements to make for various purposes. Also included are descriptions of typical gloss instruments, descriptions of measurements they make, and a bibliography on gloss. Differences between the various types of gloss are analyzed in some detail.